Heat sealable polyvinylidene chloride film coated with fatty acid salt



United States Patent 3,419,421 HEAT SEALABLE POLYVINYLIDENE CHLORIDE FILM COATED WITH FATTY ACID SALT Frank E. Eastes, Simpsonville, S.C., assignor to W. R. Grace & Co., Duncan, S.C., a corporation of Connecticut No Drawing. Filed Feb. 10, 1964, Ser. No. 343,468

8 Claims. (Cl. 117--138.8)

ABSTRACT OF THE DISCLOSURE A coating for vinyl polymer films comprising an alkali metal salt of fatty acid having 12-22 carbon atoms per molecule, inclusive, improves the heat seal strength of the films.

This invention relates to a heat sealable vinyl polymer. In another aspect the invention relates to a method for improving the seal strength of a film of vinylidene chloride polymer.

It is well known to produce containers from various thermoplastic materials such as polyolefins, polyvinyl chloride and polymers of vinylidene chloride. Generally, these containers such as plastic food bags are made in the form of extruded tubes of a thin thermoplastic material. The longitudinal edges of the sheet are lapped and are fused together under heat and pressure to form a tube. One open end is then heat sealed to form a bag. An appropriate length is cut from a roll of the flattened tubing and one end is sealed by fusion to form a bag. The food processor, after he inserts the foodstuff, may seal the bag by heat fusion, but more commonly he seals it by twisting the neck of the bag and securing it with a clip or tie.

Two methods of heating are commonly used. In the first, either one or both of the sealing jaws are heated, and the heated jaws are closed with the seam area of the plastic pinched between them. The jaws are held closed until the plastic between them has been heated by conduction and the two laminae have been fused together.

The second method induces the heat directly in the material. The jaws are not heated but are made parts of a high frequency electric circuit so that material between the jaws is heated to fusion temperature by the dielectric loss in the material when the high frequency circuit is energized.

Unfortunately, faulty seals are produced all too frequently. In general, failure does not usually occur in the film itself, but in isolated places along the seam. In a generally accepted test, known as the bag burst test, a fluid at increasing pressure is admitted into the bag until failure occurs. Generally, the two walls of the bags separate and a passage develops between them. In addition, oriented films such as biaxially oriented polymers of vinyidene chloride tend to blow out in an area immediately adjacent the seam area due to the disorientation of the film due to heating. Consequently, the great increase in strength achieved by orientation of the film is lost.

It has been found that a factor in the strength of a seal, particularly for polymers of vinylidene chloride, is the age of the film at the time the seal is made. The reason for this deterioration is not known. In general, this deterioration is not as much of a problem in polyethylene, polypropylene and other polyolefin films as it is in the polymers of vinylidene chloride. Obviously, other factors effect the seal including such obvious factors as time, temperature, area, film thickness and the like. However, independent of these factors is the age of the film from the date of extrudation to the date of sealing. The temperature during this period is also important with the higher temperatures being more adverse than the lower temperatures.

3,419,421 Patented Dec. 31, 1968 It is an object of the invention to provide an improved heat sealable polymer of vinylidene chloride.

It is another object of the invention to provide a method for improving the heat scalability of polymers of vinylidene chloride.

These and other objects of the invention will be readily apparent to those skilled in the art from the accompanying description and appended claims.

These objects are broadly accomplished by coating a polymer of vinylidene chloride on at least one heat sealable surface with an alkali metal salt of a fatty acid having 12 to 22 carbon atoms per molecule, inclusive. These additives are referred to herein as soaps.

It is known to coat the surface of thermoplastic objects to be heat sealed with antitack agents, antistatic agents, slip agents and the like. One convenient method of doing this, particularly in the production of tubing or film, employs a so called sock. The molten polymer is extruded through an annular die and the extrudate, which is in the form of a tube, is passed downwardly from the die through a pair of nip rolls. A head of a circulating cooling liquid, known as a sock, is maintained within the tube between the nip of said rolls and the face of the die in order to supercool the tube. Dispersions or solutions of antitack agents, antistatic agents, slip agents, wetting agents and the like may be included in this cooling liquid. Further details of this process are described in British Patent No. 849, 070.

It has now been surprisingly found that if a soap is included in the sock as an aqueous solution, or in conjunction with other sock components, that a heat scalable surface is provided which has increased seal strength, as compared to a surface which has not been treated with the soap.

After the extruded tubing has solidified, it is known practice to orient the film by cold working such as stretching or by the formation of the bubble. The bubble is then deflated to form a tube from which a film suitable for heat sealing to form a container is prepared. Orientation, either monoaxially or biaxially, of the film is within the scope of the invention but the invention is not so limited.

The treating agent employed to increase the seal strength of the polymer is preferably a water soluble alkali metal salt of a fatty acid having 12 to 22 carbon atoms per molecule, preferably 14 to 20 carbon atoms per molecule, inclusive. In general, the treating agent is a sodium or potassium soap formed from either a saturated fatty acid or an unsaturated fatty acid or mixtures thereof. Suitable saturated fatty acids including lauric, myristic, palmitic, stearic, arachidic, behenic acids and the like. Suitable unsaturated fatty acids include A -dodecylenic, palmitoleic, oleic, picinoleic, vaccenic, livoleic, electearic, parinaric, gadoleric, arachidonic, cetoleic acids and the like. In addition, the soaps may be formed from fats or oils containing the above, including coconut oil, babassu, palm kernel, palm, olive, castor, peanut, rape and the like.

It is preferred that the soap be soluble in water. However, any carrier fluid may be employed for depositing the soap onto the surface of the polymer so long as it is not detrimental to the polymer itself. It would appear that the length of the carbon chain of the fatty acid is not of significance. Sodium and potassium salts are preferred and the presence of various other cationic elements introduced with the soap into the composition are not detrimental.

The dispersion or solution, preferably in aqueous form, is applied to the polyolefin structure or surface in any convenient manner, such as dipping, spraying, brushing, roll coating, gravure coating, and the like, preferably at a temperature of about 60 to F. The excess aqueous coating solution may be removed by squeeze rolls, doctor knives. or the like although in many instances no removal is necessary. The coating composition should be applied in such an amount that there will be deposited upon drying a smooth, evenly distributed layer about 1X10 to about l 10 gram per square foot, about 1 10 to l gram per square foot being preferred. The thicktubing. The surface concentration was determined by weighing the sample, placing the contaminate on the surface in the form of a dilute solution or dispersion and reweighing after drying. The treated sample was then heat sealed to itself by a radio frequency sealer having inch ness of the applied coatin-g should be sufficient to form a 5 Teflon coated sealing jaws using a 0.2 second pulse, 40 thin, relatively transparent film spread evenly over the p.s.i. jaw pressure and an initial jaw temperature of 140 surface of the coated surface, preferably the soap is dis- F. Cooling time was 1 second after sealing. The rheostat solved in water at a concentration of 0.01 to 10.0 weight was set at 70. The power was supplied by a radio frepercent, more preferably 0.1 to 1.0 weight percent based l0 quency generator, a model 2DA produced by Lake Servon total composition weight. ice Corp, Boston, Mass.

The treating agent may be applied to one or both sides An untreated sample was heat sealed in the same of the surface but should in any case be applied to the manner. The bag burst strength of the samples were desurface to be subsequently heat sealed. The invention is termined by subjecting the thus prepared sealed bags described primarily with reference to the heat sealing of (sealed at one end) to an internal air pressure (measured a film although improvement is found with any shape of in inches of water) which was gradually increased until the polymer such as tubing or solid structures. the seam failed.

Any suitable method may be employed for heat sealing The ratio of the heat seal strength of the contaminated the two surfaces together such as impulse sealing or insamples to untreated samples was then compared. A ratio ductive heating by high frequency electrical energy. greater than 1.0 represents an improvement; less than 1.0

The treating agent is broadly applicable to the surface indicates that the contaminate was detrimental to heat treatment of a synthetic organic thermoplastic polymer sealing.

TABLE 1 Concentration, Strength Run N0. Contamination gmJit. ratio 1 0.5% polyoxyethylene esters of tall oil 7X10- 0. 85 2. 0.5% polyoxyethylene esters of tall oil 1. 7X10-1 0. 65 3. 1.5% glycerol mannitan laurate 2 4. 4X10-2 0.86 4. Polyvinyl chloride dispersion 3 2. 8 10- 0. 57 5. Sodium lauryl sulfate 5. 0X10-4 0.98 o o 7.4x10= 0. so 7- Ivory soap- 2. 5X105 1. 14 8 Ivory soap 1. 1X10'1 1. 10

l Renex 20. NNO. 3 Geon 121.

and is particularly useful for vinyl polymers including polyvinyl chloride, the polyvinylidene chloride polymers and the copolymers of vinylidene chloride and other ethylenically unsaturated monomers including the vinyl esters such as vinyl acetate, vinyl butyrate, vinyl benzoate and the like; the vinyl ethers such as vinyl ethyl ether, vinyl chloroethyl ether, vinyl phenyl ether and the like; the vinyl ketones such as vinyl methyl ketone, vinyl phenyl ketone and the like; the vinylidene halides such as vinylidene chloride, l-fluoro l-chloroethylene and the like; the acrylic compounds such as acrylonitrile, chloroacrylonitrile, methyl acrylate, methyl methacrylate, 2-ethylhexy1 acrylate and the like; the allylic compounds such as alkylidene diacetate, chloroallylidene diacetate and the like; and other mono-unsaturated compounds.

The term polymer as used herein includes homopolymers, copolymers, terpolymers, block copolymers and the like. Particularly preferred are the vinylidene chloride copolymers which are predominately vinylidene chloride, preferably 20 to 80 percent vinylidene chloride, with the comonomer preferably being vinyl chloride.

The invention is best illustrated by the following examples.

EXAMPLE I The effect of surface contamination on seal strength was determined on 1.5 mil Saran polymer tubing having the following composition:

Dow 025 Saran Resin, a copolymer of vinylidene chloride and vinyl chloride.

An aqueous dispersion of a number of materials was placed on a heat scala le surface of the Saran polymer The above data show that conventional sock components (Runs 1-4) are actually detrimental to the heat seal strength of the polymer film. Surprisingly, Ivory soap (Runs 7-8) actually improves the heat seal strength whereas another common detergent, sodium lauryl sulfate (Runs 5-6), is quite detrimental.

EXAMPLE II Saran polymer having the formulation shown in Example I was extruded vertically downward at 325 F. forward cylinder temperature through a 2 /2 inch annular die having a rnil gap. The extruded tube was pinched off by a set of squeeze rolls below the die. These rolls also seuved to pull the tubing away from the die and stretch it down to the desired gauge.

The tubing between the die and the pinch rolls was substantially filled with a liquid known as a sock. The composition of this sook was varied for each of the following runs to determine the elfect of different soaps on the ultimate heat seal strength of the polymer. For control purposes the control sock composition comprised:

Wt. Percent Finely divided polyvinyl chloride resin (Geon 121 25 Glycerol mannitan laurate, non-ionic detergent (NNO) 1.03 Polyoxyethylated tall oil (16 moles ethylene oxidea non-ionic detergent) 0.62 Emulsion of polydialkylsiloxane, an antifoa-m emulsion (Antifoam 60) 0.2

Water 73.15

Several series of runs were made comparing the control sock with difierent sock compositions. For each series (Runs 8, 9, 10, 11 in Table H) a different control was run. For each series seals were made at a rheostat setting of 70 (except Runs llA-B-C-D which was 80) within EXAMPLE III The elfect of different seal temperatures and prolonged aging was determined on the samples prepared in Example II. The seals were made as described in Example 11 24 hours after extrusion. Pnor to seahng these fresh 5 with the samples aged at room temperature (about 75 F.) films were held in a cold room at P. so that no for the time shown 1n the followlng table.

TABLE III Seal Strength vs. Run N o. Sock Composition, RF Power Fresh Weeks Aging wt. percent Setting 12A Control 70 109 97 88 76 76 (Same as Run 8A) 90 116 98 95 84 82 77 110 112 98 96 85 83 80 12B 1.0% Ivory soap 115 117 121 116 84 72 (Same as Run 8E) 90 123 118 123 115 84 85 110 127 124 120 111 91 93 13A Control 70 114 102 95 84 79 68 (Same as Run 9A) 90 119 98 93 90 84 81 110 120 99 97 94 87 86 13B 0.825% Na 70 125 113 112 101 100 85 Stearate, 16.5% 90 127 115 113 106 105 94 PVC, 1% antifoam 110 126 116 113 109 107 103 (Same as Run 9B) 14A Control 70 104 89 85 77 73 53 90 112 91 88 84 81 110 110 91 90 86 84 14B 0.1% Castile 70 120 116 106 94 94 79 Soap 130 119 111 105 102 88 Control sock 110 133 116 112 105 105 99 15A Control 70 111 107 104 90 65 (Same as Run 10A) 90 121 111 108 101 97 86 110 118 111 108 104 97 15B Control+4% 70 115 121 117 103 96 Koleate 90 129 116 122 110 106 110 132 121 116 115 115 16A 70 109 100 93 85 84 68 90 116 98 99 89 89 81 110 117 102 102 94 91 89 16B 25% PVC, 2% 70 107 98 98 88 88 49 Koleate, 15%-- 90 111 102 100 98 96 86 Propylene glycol 110 118 101 104 100 102 92 deterioration would take place. For these fresh" films the bag burst test was made 24 hours or more after sealing. The aged samples were aged for the specified time at 45 C. Again the bag burst test was made 24 hours or more after the seals were made. There is every indication that after 24 hours aging there is no reduction in seal strength measured from the time the seal is made but that the time from extrusion until seal is an important factor in seal strength although this is negligible at the lower temperatures of the cold room. The results are shown in the following table.

These data demonstrate that soaps are efiective in prolonging the ability of the film to form strong seals over a wide range of sealing temperatures even when stored for as long as 9 weeks at 75 F.

45 I claim:

1. A vinyl polymer having improved seal strength TABLE II Seal Inches H2O Run No Sock Composition wt. percent Fresh Strength, 8 days 4 days Aging Aging 8A Control 106 10% PVC +0.5% Ivory soap 2 100 l I do 104 25% PVC 0.5% Ivory soap 104 1.0% Ivory soap 96 Control 103 0.825% Na stearate, 16.5% PVC, 1% antiloam 3 107 1.10% Na myristate, 16.5% Zn stearate, 1% antii'oam. 92

Control 110 Control+1% K oleate (unrefined) 117 Control+4% K oleate (unrefined) 120 Control-91% coconut oil soap 4 123 Control+4% coconut oil soap 92 Control 121 Control+4% K oleate (unrefined) 123 4% K oleate+25% PVC+30% propylene glycol 115 11D 4% coconut oil soap +8% PVO+72% propylene glycol 102 1 Polyvinyl chloride resin-Geon 121. 2 Proctor and Gamble Corp.

3 Polydialkylsiloxane, Antifoam 60.

4 Kokobace R.

* Unless otherwise specified the remainder of the sock composition was water. TEmployed a double sock technique wherein some sock was permitted to pass the first set of rolls.

70 coated with an alkali metal salt of a fatty acid having 12-22 carbon atoms per molecule, inclusive.

2. A polymer of predominately vinylidene chloride having improved seal strength coated on at least one surface with an alkali metal salt of a fatty acid having 75 12-22 carbon atoms per molecule, inclusive.

3. A film of a copolyrner of 2080% vinylidene chloride having improved seal strength coated on at least one heat sealable surface with a soap evenly distributed between 1X10" gm. sq. ft. and 1X10 gm. sq. ft., said soap comprising an alkali metal of a fatty acid having 1222 carbon atoms per molecule, inclusive.

4. The film of claim 3 wherein said soap comprises sodium stearate.

5. The film of claim 3 wherein said soap comprises potassium stearate.

6. The film of claim 3 wherein said soap comprises potassium oleate.

7. A method for improving the seal strength of a film of a polymer of vinylidene chloride to be heat sealed to itself comprising coating at least one of the surfaces to be sealed with an aqueous soap solution, said soap being dissolved in said aqueous solution at a concentration between 0.01 and 10.0 Weight percent and said soap com,- prising an alkali metal salt of a fatty acid having 12-22 carbon atoms per xolecule; and drying said coating to volatilize substantially all of said water before heat sealmg.

8. A film of a polymer of vinylidene chloride having a coating on a heat scalable surface, said coating being evenly distributed between 1 10 gram per square foot and 1X10 gram per square foot on said surface, and said coating being the residue deposited by drying a solu- 8 tion comprising between 0.1 to 10 Weight percent of a soap comprising an alkali metal salt of a fatty acid having 12 to 22 carbon atoms per molecule incluisve, between 10 to 25 weight percent of finely divided polyvinyl chloride resin, between 1.0 to 1.65 percent non-ionic detergent, a silicon anti-foaming agent, and water.

References Cited UNITED STATES PATENTS 2,520,959 9/1950 Powers 26023 3,068,186 12/1962 Paulus et al 26029.6 3,236,675 2/ 1966 Hermitte et al. 1177 3,252,826 5/1966 Salzinger 117-1388 3,255,132 6/1966 Reinecke 26023 3,291,768 12/1966 Pfluger et a1 260-29.6

FOREIGN PATENTS 849,070 9/ 1960 Great Britain.

DONALD E. CZAJA, Primary Examiner.

R. A. WHITE, Assistant Examiner.

US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,419,421 December 31, 1968 Frank E. Eastes It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Celumn 7, line 5, after "alkali metal" insert salt Signed and sealed this 21st day of April 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr. E. JR.

Attesting Officer Commissioner of Patents 

